1. Field of Invention
The present invention relates to improvements in metal-air fuel cell battery (FCB) systems and devices, and more particularly to a movable anode design for use in such systems and devices in order to obtain improvements in charging and discharging of the anode structures employed therein.
2. Brief Description of the Prior Art
The use of metal-air FCB systems and devices for electrical power generation offers great promise for the future of mankind.
U.S. Pat. No. 5,250,370 to Faris discloses an exemplary metal-air FCB system. According this metal-air FCB system design, a bifunctional air electrode is arranged on one side of a rotating anode structure, for carrying out discharging and recharging operations.
Also, WIPO Publications WO 99/18627, WO 99/18628 and WO 99/18620 by Applicant disclose various types of metal-air FCB systems employing moving anode and cathode structures which can be used to generate electrical power for a variety of applications using a metal, such as zinc, and air as fuel. However despite the incredible advances that such metal-air FCB systems represent to the electrical power industry, there nevertheless remain a number technical problems which limit the recharging performance of such electrical power generation systems. In large part, such problems are due to a number of factors, including: (i) the deformation of shape/geometry of the anode structure; (ii) densification of the anode structure; and (iii) formation dendrites on the anode structure which reach through the separator, touch the air electrode, and eventually short out the cell. In general, such anode related problems have limited the operational life of prior art rechargeable FCB systems and devices.
Hitherto, prior art solutions to these problems have typically involved decreasing the current density of the anode structure during both discharging and recharging operation, as well as the depth of discharge thereof. Both of these side-effects have severely limited the energy and power density characteristics of prior art metal-air FCB systems. Thus, in prior art metal-air FCB systems and devices, there has been a serious trade off between achieving high energy/power densities and good charging characteristics.
Also, when using prior art techniques, it has not been possible to construct a bifunctional air electrode for use in metal-air FCB systems which operates in an efficient manner during both recharging and discharging operations.
Another problem with prior art FCB systems is that the anode structure undergoes deformation during the lifetime of the system. When the anode undergoes deformation during each recycling/recharging operation, the capacity of the system decreases significantly, and shorting problems often occur.
One attempt to solve the anode deformation problem has been to use a reticulated sponge-like zinc anode in order to increase the surface area of the zinc (and thus decrease the current density therewithin). However, the lowered current density decreases the energy density of the FCB system. Also, the use of a reticulated sponge-like zinc anode does not prevent the growth of dendrites on the anode.
Attempts by others have been made to limit dendrite growth on reticulated zinc anode structures. One approach has involved using a chemically inert coating on the exterior of the anode structure. While this reduces dendrite growth, the loss of the anode area lowers the capacity of the cell.
Prior art attempts to reduce anode deformation have involved the use of a pump to circulate the electrolyte. By continually stirring the electrolyte within the cell, a more uniform distribution of zinc ions in solution will result. A uniform mixture of zinc ions in the electrolyte can greatly reduce anode shape deformation over repeated cycling.
U.S. Pat. No. 3,663,298 discloses a method of reducing anode shape deformation and dendrite growth. According to this prior art approach, zinc pellets and electrolyte are used to fill about ⅔ of the volume of a circular rotating drum, on the walls of which the air electrode is formed. The drum rotates during discharging and recharging operations, and the zinc particle bed continually mixes within the cell. Because the particles move freely, fresh zinc continually and evenly is exposed to the air electrode. By evenly depositing zinc during recharging operations, a longer discharge life can be achieved at higher current densities.
U.S. Pat. No. 3,663,298 discloses that the use of a rotatable electrode improves the recharging characteristics of metal-air FCB systems. As disclosed, this technique enables repeated recharging and discharging a rotating electrode at rates up to 100 mA/cm2. Conventional zinc electrodes do not ordinarily withstand recharge rates in excess of 20 mA/cm2 on repeated cycling without rapid failure by dendrite shorting. The high recharging rates were possible because the continual movement of the particle bed provided for a smooth, dendrite free, zinc coating on the pellets.
While rotatable electrode concept of U.S. Pat. No. 3,663,298 improved upon conventional zinc/air FCB technology, it required the use of an inefficient bifunctional air electrode.
Bifunctional air electrodes have very low cycle numbers because the electrode has to be used both for charging and discharging. Bi-functional electrodes are inefficient for discharging because they must simultaneously be optimized for recharging. In addition, prior art bi-functional electrodes are generally thick and heavy to slow down degradation processes. Their significant weight and size reduces the energy density of the system. In the past, others have tried using many different catalysts and different electrode structures to make bifunctional air electrodes with improved performance characteristics, but the lives of prior art rechargeable zinc-air FCB systems have been severely limited.
In the Sony Corporation publication entitled xe2x80x9cFuel Cell and their Applicationxe2x80x9d published in 1996 (at pg. 160), there is disclosed a rechargeable metal/air FCB system design employing a third electrode. This FCB system comprises a zinc anode sandwiched between one recharging air electrode and one discharging air electrode. This prior art approach to metal-air FCB construction sought to eliminate the need for a bifunctional air electrode. According to the approach, the zinc anode would be discharged from one side and recharged from the opposite side, while using different discharging an d recharging electrodes that are optimized for their independent functions.
While Sony""s zinc/air cell was an improvement on the bifunctional air electrode, the zinc anode could only be discharged form one side, thus reducing the power capabilities of the cell by 50%. Further, the zinc anode is charged from the side where it was discharged the least, which decreases the efficiency of the system.
Another problem presented by the Sony design is that the anode has to be a porous structure so that the electrolyte can flow from the discharge side to the recharge side to provide ions in solution from discharging in order to recharge again.
Thus, there is a great need in the art for an improved way of and means for producing electrical power using a rechargeable metal-air FCB system having high energy density, high power density, and good rechargeability, while overcoming the shortcomings and drawbacks of prior art technology.
Accordingly, it is a primary object of the present invention is to provide an improved rechargeable metal-air FCB system having high energy density, high power density, and good rechargeability.
Another object of the present invention is to provide a metal-air FCB system having metal anodes which do not undergo any significant shape deformation (i.e. change) during charging and discharging operations, in order to ensure a longer battery life.
Another object of the present invention is to provide such a metal-air FCB system, wherein dendrite formation on metal anodes is controlled.
Another object of the present invention is to provide such a metal-air FCB system having a fast recharging capability.
Another object of the present invention is to provide a metal-air FCB system having an increased turnaround efficiency for recharging the anode structures employed within such a system. Another object of the present invention is to provide such a metal-air FCB system, wherein the anode structure is realized in the form of a disc structure on which metal fuel material is supported.
In accordance with one aspect of the present invention, a novel metal-air FCB system is provided, wherein a movable anode is sandwiched between two stationary air electrodes. The air electrodes are divided into a recharge air electrode portion, to maximize recharging of the anode structure, and a discharge air electrode portion for maximizing the discharging of the anode structure. The anode structure is moved either rotationally or linearly with respect to the air electrodes; exposing portions of the anode alternately to the recharging and discharging portions of the air electrode. Electrolyte fills the space between the air electrodes and the movable anode structure.
Previous solutions to the metal/air rechargeability problem could only increase cycle life at the expense of decreasing energy and power densities. In marked contrast, the movable anode FCB system of the present invention has both increased cycle life and discharge performance. Recycleability is increased for the following reasons.
The recharging electrode of the present invention is intended solely for recharging operations, and thus there is no need to use a bifunctional air electrode. An air electrode intended solely for recharging operations will not limit the lifetime of the cell. The cycle life of the cell will be limited by the lifetime of the anode structure.
The electrolyte in each cell of the FCB system is continually stirred during recharging operations. The stirring action creates an even distribution of zinc ions in solution. This results in an even plating on the zinc anode, which greatly reduces the anode shape deformation.
In accordance with the present invention, the anode structure of the FCB system is continually moving during recharging operations, which greatly reduces dendrite growth and anode shape deformation. These phenomena occur because of an uneven electric field distribution on the zinc surface. If one spot has a slightly higher electric field than another, this spot will continually attract zinc ions. However, with the anode moving, the point of peak electric field will be changing positions and moving in and out of the recharging area; reducing the chances of localized buildup. If the movement alone does not stop dendrite growth, they can be removed mechanically by a stationary wiper attached to the air electrode holder. As the anode moves past this wiper, the dendrites will be smoothed out or scraped off.
The recharging air electrode of the present invention can be several times larger than the discharging air electrode. This will allow for fast recharging operations, while still using a low current density. In prior art fixed anode FCB systems, the only way to decrease charging time was to increase charging current density. High charging current density significantly decreases cycle life, and turnaround efficiency which is defined as a ratio of the power output of a cell and the power required to charge it. A decreased turnaround efficiency implies less electrical power is required to charge the cell.
In the metal-air FCB system of the present invention, high energy density is obtained for the following reasons.
The design of the movable anode FCB system of the present invention allows the weight of the cell to be dominated by the metal anode. Consequently, the energy density of the cell has the ability to approach that of the metal anode.
The movable anode FCB system of the present invention has the ability to increase energy density by increasing the anode""s depth of discharge (DOD). This means a greater percentage of the anode can be discharged. The movable Anode FCB system can increase ODD because it limits passivation. An anode will passivate if too much current is drawn from it in too short a time. Passivation can be substantially decreased if the anode is discharged intermittently (in other words, allowing the anode to rest between discharging can eliminate passivation). The movable anode FCB system of the present invention will do exactly that. As each anode section moves away from the discharge electrode, it has a rest period before it is discharged some more. In addition, because of the moveable anode""s unique recharging capabilities, the anode can afford to be discharged to a high DOD without sacrifice of cycle life whereas most recharging batteries must limit their DOD to ensure reasonable recycleability.
The discharging air electrode in the FCB system of the present invention is intended solely for carrying out discharging operations, and therefore, there is no need for the use of bi-functional air electrodes. This implies that the discharge electrode can be optimized exclusively for discharging.
Preferably, the electrolyte in each cell of the metal-air FCB system of the present invention is continually stirred during discharging operations. By stirring the electrolyte, it is possible to increase its capacity, implying that less electrolyte is needed, which translates to a higher energy density.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.